This invention relates to a method of measuring glucose concentration using a glucose sensor.
Various types of glucose sensors having a glucose oxidase-immobilized film provided on the ion-sensing section of an ion-sensitive field-effect transistor which detects pH change in sample solutions have so far been reported (Biophysica Biochimica Acta, Vol. 320, pp. 529-534, 1973; and Analytical Chemistry, Vol. 57, pp. 1917-1925, 1985). The principle of these sensors is to measure glucose level based on the pH change to be brought about by the catalytic action of glucose oxidase which oxidizes glucose to form gluconic acid. The reaction scheme that gluconic acid is formed from glucose is as follows: EQU Glucose+O.sub.2 .fwdarw.Glucono-.delta.-lactone+H.sub.2 O.sub.2 ( 1) EQU Glucono-.delta.-lactone+H.sub.2 O.fwdarw.Gluconic acid (2)
The reaction (1) takes place catalyzed by glucose oxidase. The reaction (2) proceeds spontaneously or catalyzed by gluconolactonase.
In the measurement of glucose using a glucose sensor of the type described above, the sensor is first dipped in a predetermined amount of buffer containing no glucose to measure a background output, and then a predetermined amount of sample solution is added to the buffer to measure a response output. The difference between the background output and the response output corresponds to the glucose concentration. This can be explained below referring to the attached drawings.
FIG. 1, (a) shows schematically a constitution of a system used for the glucose measurement by a conventional glucose sensor; FIG. 1(b) explains the measuring method using the same. FIG. 1(c) shows an output curve of the sensor corresponding to FIG. 1(b). The glucose sensor 33 is first dipped in a predetermined amount of buffer 31 to measure a sensor output. This output value is the background output. Next, a predetermined amount of sample solution 34 is added to the buffer 31 using a pipette 35, whereupon the output level of the sensor 33 increases. This is the response output. The output of the sensor in a series of process changes is shown in FIG. 1(c). Glucose concentration of the sample solution can be determined from the difference between the response output and the background output.
As described above, in the conventional glucose measuring method a buffer is inevitable and yet the buffer and sample solution must be measured correctly. Accordingly, in order to establish a system using a glucose sensor, the system requires a buffer and a measuring device, leading to increase in the size of the system, disadvantageously.